1. Field of the Invention
The present invention relates to an optical three-dimensional shape measuring apparatus and, more particularly, to such apparatus of pattern irradiation type.
2. Related Background Art
In the conventional optical three-dimensional shape measuring apparatus of the pattern irradiation type, a predetermined pattern is projected, through an irradiating optical system onto the surface of a specimen to be measured, then the projected pattern is observed by an observing optical system, and the surface shape of the specimen is measured, based on the variation of the observed projected pattern. Stated differently, the conventional optical three-dimensional shape measuring apparatus of the pattern irradiation type utilizes non-contact surface measurement based on a pattern projection method.
This method can measure the surface shape of any object, except for an object of which surface reflectivity varies significantly by the incident angle, such as an optical mirror surface (an object generating mostly normally reflected light but little scattered light) or a transparent object scarcely generating reflected (scattered) light.
In the optical measurement mentioned above, the projected pattern has to be made finer, in order to improve the precision of shape measurement, namely to improve the resolving power of the measurement. In general, the extent of condensing of light depends on the numerical aperture (NA) of the optical system, and the projected pattern becomes finer as the numerical aperture increases. However a larger numerical aperture reduces the depth of focus of the optical system.
As explained in the foregoing, in the conventional optical three-dimensional shape measuring apparatus, the depth of focus of the optical system becomes smaller when the surface shape of the specimen is to be measured with a higher precision or a higher resolving power. Consequently if the surface of the specimen is relatively flat (planar), a highly precise shape measurement is possible over the entire projection area (corresponding to the area of observation) of the optical system in a single operation. On the other hand, if the surface of the specimen is irregular, in portions within the projection area of the optical system where the focusing does not reach a desired level, the shape measurement itself becomes impossible, not to speak of the precision thereof.
In the conventional optical three-dimensional shape measuring apparatus, therefore, when measuring the surface shape of a general specimen with an unflat surface with a high precision, it is required to employ an extremely simple irradiation pattern consisting for example of a single spot, and to repeat the measurement while maintaining precise positioning (namely focusing) with the optical system in an extremely narrow area corresponding to such small projection pattern.